Metalworking fluid used in metalworking is generally categorized into oil-type (oil-based) fluid and water-type (water-based) fluid, the latter of which is more frequently used because such water-based fluid is excellent in cooling capabilities and infiltration capabilities and free from a risk of causing a fire. Known water-based fluid includes: emulsion-type fluid prepared by mixing a surfactant with oil-based base oil such as a mineral oil; soluble-type fluid which contains an increased content of surfactant; and solution-type fluid, the main component of which is a water-soluble lubricating agent such as a polyalkylene glycol. Water-soluble solution-type fluid not containing a mineral oil is frequently used when cooling capabilities is significant, e.g., in grinding. Known solution-type fluid is a water-based lubricating composition prepared by mixing an alkanolamine fatty acid salt with a polyoxyalkylene glycol (see Patent Literature 1).
However, since a long-lasting grinding stone using superabrasives (diamond, CBN) has been recently widespread, more lubricity is being demanded for solution-type fluid. Accordingly, water-soluble metalworking fluid containing mono-carboxylic acid or di-carboxylic acid having carbon atoms of 6 to 10 and a block polyalkylene glycol of (PO)-(EO)—(PO) type is proposed (see Patent Literature 2). According to this fluid, a solution-type fluid excellent in lubricity and antifoaming capabilities can be provided.